Driver circuitry for piezoelectric transducers

ABSTRACT

The present disclosure relates to driver circuitry for driving a piezoelectric transducer. The circuitry comprises: a power supply; a reservoir capacitance; switch network circuitry; and control circuitry. The control circuitry is configured to control operation of the switch network circuitry so as to charge the reservoir capacitance from the power supply and to transfer charge between the reservoir capacitance and the piezoelectric transducer.

FIELD OF THE INVENTION

The present disclosure relates to driver circuitry for piezoelectrictransducers.

BACKGROUND

Piezoelectric transducers are increasingly being seen as a viablealternative to transducers such as speakers and resonant actuators forproviding audio and/or haptic outputs in devices such as mobiletelephones, laptop and tablet computers and the like, due to their thinform factor, which may be beneficial in meeting the demand forincreasing functionality in such devices without significantlyincreasing their size. Piezoelectric transducers are also increasinglyfinding application as transducers for ultrasonic sensing andrange-finding systems.

Piezoelectric transducers can be voltage-driven. However, when driven byvoltage piezoelectric transducers exhibit both hysteresis and creep,which means that when driven by voltage the displacement of apiezoelectric transducer depends on both the currently-applied voltageand on a previously-applied voltage. Thus, for any given driving voltagethere are multiple possible displacements of the piezoelectrictransducer. For audio applications this manifests as distortion.

One way of reducing hysteresis and creep and the associated problems ina piezoelectric transducers is to drive the transducer with chargeinstead of voltage. When driven with charge, the displacement of thepiezoelectric transducer varies with the charge applied.

FIG. 1 is a schematic illustration of circuitry for driving apiezoelectric transducer with charge. As shown generally at 100 in FIG.1, charge drive circuitry 102, which may be charge pump circuitry, forexample, may receive an electrical input signal (e.g. an input audio orultrasonic signal or haptic waveform) from upstream circuitry (notshown) such as amplifier circuitry, and drive a piezoelectric transducer104 to cause the piezoelectric transducer 104 to produce an audible,ultrasonic or haptic output based on the electrical input signal.

It is desirable to transfer charge to the piezoelectric transducer withhigh efficiency, to minimise power consumption, particular where thepiezoelectric transducer and the drive circuitry 102 are part of abattery powered device such as a mobile telephone or the like. Whentransferring charge to the piezoelectric transducer 104, charge can bedrawn from the power supply (e.g. a battery). However, when reducingcharge on the piezoelectric transducer 104, transferring charge back tothe power supply is not desirable, particularly where the power supplyis a battery, as this can shorten the usable life of the battery.

Additionally, for thin piezoelectric transducers which include only asmall number (e.g. 1) of layers of piezoelectric material, a relativelylarge voltage (e.g. 70 volts) is required to drive the piezoelectrictransducer 104 to cause displacement of the transducer. If the drivecircuitry 102 is powered by a low voltage power supply (e.g. a 3 voltbattery), then a mechanism to increase the voltage available at theoutput of the drive circuitry 102 is required. Typically a switchingconverter would be used to boost the voltage. The duty cycle D of aswitching converter for a given voltage gain g is given by me equation

$D = {\frac{g}{1 + g}.}$

Thus, for a voltage gain of 23.3 required to boost a 3 v supply voltageto a 70 v output voltage, a duty cycle of 96% would be required, if asingle converter were used. As will appreciated by those of ordinaryskill in the art, this is difficult to achieve in practice. Using two ormore converters to boost the supply voltage in multiple stages wouldreduce the duty cycle required at each converter, but would requiremultiple inductors (one for each converter) and many switches.

SUMMARY

According to a first aspect, the invention provides circuitry fordriving a piezoelectric transducer based on an input signal, thecircuitry comprising:

-   -   a power supply;    -   a reservoir capacitance;    -   switch network circuitry; and    -   control circuitry configured to control operation of the switch        network circuitry so as to charge the reservoir capacitance from        the power supply and to transfer charge between the reservoir        capacitance and the piezoelectric transducer.

The circuitry may further comprise one or more inductors. The controlcircuitry may be configured to control operation of the switch networkcircuitry to transfer charge between the reservoir capacitance and thepiezoelectric transducer via one of the one or more inductors.

The control circuitry may be configured to control operation of theswitch network circuitry to transfer charge from the power supply to thereservoir capacitance via one of the one or more inductors.

Alternatively, the control circuitry may be configured to controloperation of the switch network circuitry so as to transfer chargedirectly between the reservoir capacitance and the piezoelectrictransducer.

A capacitance value of the reservoir capacitance may be variable.

The control circuitry may be further configured to control operation ofthe switch network to transfer charge from the power supply to thereservoir capacitance based on an indication of a level of the inputsignal.

The circuitry may further comprises monitoring circuitry configured tomonitor a level or magnitude of the input signal and to output a signalindicative of the level or magnitude of the input signal to the controlcircuitry.

The circuitry may further comprise look-ahead circuitry configured toreceive the input signal and to output a signal indicative of the levelof the input signal to the control circuitry.

The circuitry according may further comprise envelope detector circuitryconfigured to receive the input signal and to output a signal indicativeof an envelope of the input signal to the control circuitry.

The switch network circuitry may be configured for coupling to a singleterminal of the piezoelectric transducer.

Alternatively, the switch network circuitry may be configured to becoupled to first and second terminals of the piezoelectric transducer.

The power supply may be configured to provide an output voltage thatvaries according to a level of the input signal.

The power supply may be configured to provide an output voltage that isgreater than a voltage supplied to the power supply by a power source.

The power supply may comprise a switching power supply, for example.

The control circuitry may be configured to receive a feedback signalindicative of a level of charge of the piezoelectric transducer.

The feedback signal may be based on a voltage across the piezoelectrictransducer.

The circuitry may comprises one or more inductors, and the controlcircuitry may be configured to control operation of the switch networkcircuitry to transfer charge between the reservoir capacitance and thepiezoelectric transducer via one of the one or more inductors, and thefeedback signal may be based on a current through the one of the one ormore inductors.

The input signal may comprise an audio signal, a haptic waveform or anultrasonic signal.

The circuitry may further comprise commutator circuitry coupled to theswitch network circuitry, the commutator circuitry being configured toselectively couple a first or a second terminal of the piezoelectrictransducer to an output of the switch network circuitry.

According to a second aspect, the invention provides circuitry fordriving a piezoelectric transducer based on an input signal, thecircuitry comprising:

-   -   a power supply;    -   a reservoir capacitance;    -   switch network circuitry; and    -   control circuitry configured to control operation of the switch        network circuitry so as to charge the reservoir capacitance from        the power supply based on an indication of the input signal.

According to a third aspect, the invention provides circuitry forestimating a level of charge on a piezoelectric transducer, thecircuitry comprising:

-   -   an inductor for transferring charge to the piezoelectric        transducer from a charge source;    -   control circuitry configured to control transfer of charge to        the piezoelectric transducer based on an input signal,    -   wherein the control circuitry is configured to receive an        indication of a current through the inductor and an indication        of a voltage across the piezoelectric transducer,    -   and wherein the control circuitry is configured to estimate the        level of charge on the piezoelectric transducer based on:        -   the indication of the voltage across the piezoelectric            transducer when a frequency of the input signal is within a            first range, and        -   the indication of the current through the inductor when the            frequency of the input signal is within a second range.

According to a fourth aspect, the invention provides integratedcircuitry comprising the circuitry of any of the first to third aspects.

According to a fifth aspect, the invention provides a device comprisingthe circuitry of any of the first to third aspects.

The device may comprise, for example, a mobile telephone, a tablet orlaptop computer, a smart speaker, an accessory device, headphones,earphones or earbuds.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, strictly by way ofexample only, with reference to the accompanying drawings, of which:

FIG. 1 is a schematic diagram illustrating the concept of driving apiezoelectric transducer with charge;

FIG. 2a is a schematic diagram illustrating circuitry for transferringcharge between a power supply, a reservoir capacitance and apiezoelectric transducer;

FIGS. 2b-2g illustrate the operation of the circuitry of FIG. 2 a;

FIG. 3a is a schematic diagram illustrating alternative circuitry fortransferring charge between a power supply, a reservoir capacitance anda piezoelectric transducer;

FIGS. 3b-3h illustrate the operation of the circuitry of FIG. 3 a;

FIG. 4 is a schematic diagram illustrating further circuitry fortransferring charge between a power supply, a reservoir capacitance anda piezoelectric transducer;

FIG. 5 is a schematic diagram illustrating circuitry for transferringcharge between a power supply, a reservoir capacitance and apiezoelectric transducer; and

FIG. 6 is a schematic diagram illustrating commutator circuitry that canbe used to provide bipolar drive of a piezoelectric transducer.

DETAILED DESCRIPTION

FIG. 2a is a schematic representation of a system 200 for driving apiezoelectric transducer 210.

The system 200 includes a reservoir capacitance 220 for storing charge,an inductor 230 and a switch network 240 (in this example comprisingfirst to fifth controllable switches 242-250, which may be, for example,MOSFET devices) for transferring charge between the reservoircapacitance 220 and the piezoelectric transducer 210. The reservoircapacitance 220 is shown in FIG. 2a as a single capacitor, but it willbe appreciated that the reservoir capacitance 220 may alternatively beprovided by a plurality of capacitors coupled together.

The system 200 also includes a power supply 260 for selectivelyproviding charge to the reservoir capacitance 220. In some examples thepower supply 260 may comprise a battery. In other examples the powersupply 260 may comprise power supply circuitry that receives power froma power supply such as a battery.

Although the system 200 is shown as including only a single inductor 230(and this may be preferable, to minimise the number of externalcomponents and thus reduce the cost and space requirements of the system200), in some examples there may be more than one inductor. For example,a first inductor may be provided for transferring charge from the powersupply circuitry 260 to the reservoir capacitance 200 and a secondinductor may be provided for transferring charge from the reservoircapacitance 220 to the piezoelectric transducer 210.

The first switching device 242 is coupled between an output of the powersupply 260 and a first terminal the inductor 230.

The second switching device 244 is coupled between the first terminal ofthe inductor 230 and a ground/reference voltage supply rail.

The third switching device 246 is coupled between a second terminal ofthe inductor 230 and the ground/reference voltage supply rail.

The fourth switching device 248 is coupled between the second terminalof the inductor 230 and a first terminal of the reservoir capacitance220. A second terminal of the reservoir capacitance is coupled to theground/reference voltage supply rail.

The fifth switching device 250 is coupled between the first terminal ofthe inductor 230 and a first terminal 212 of the piezoelectrictransducer 210. A second terminal 214 of the piezoelectric transducer210 is coupled to the ground/reference voltage supply rail.

The system 200 further includes control circuitry 270, operable tocontrol the switching devices 242-250 to control the transfer of chargebetween the power supply 260, the reservoir capacitance 220 and thepiezoelectric transducer 210, as will now be explained with reference toFIGS. 2b -2 g.

On start-up of the system 200 (or a host device incorporating the system200), charge is transferred from the power supply 260 to the reservoircapacitance 220 to raise a voltage across the reservoir capacitance 220to a level that is suitable for driving the piezoelectric transducer210.

In a first phase of a charging process (illustrated in FIG. 2b ), thefirst and third switches 242, 246 are closed, as indicated by the dashedlines, in response to appropriate control signals transmitted by thecontrol circuitry 270. This creates a current path (indicated by thedashed arrow) though the inductor 230. As current flows through theinductor 230 a magnetic field develops around it, storing energy.

In a second phase of the charging process (illustrated in FIG. 2c ), thefirst and third switches 242, 246 are opened and the second and fourthswitches 244, 248 are closed, as indicated by the dashed lines in FIG.2c , again in response to appropriate control signals transmitted by thecontrol circuitry 270. The magnetic field around the inductor 230collapses, inducing a current which flows from the inductor 230 to thereservoir capacitance 220, thereby charging the reservoir capacitance220.

The first and second phases are repeated until the voltage across thereservoir capacitance 220 has increased to a level that is suitable fordriving the piezoelectric transducer 210, as determined by the controlcircuitry 270 based on a feedback signal received from the piezoelectrictransducer 210. Once the reservoir capacitance 220 has been charged upto the desired level the first switch 242 is opened, thus decoupling thepower supply 260, such that the piezoelectric transducer 210 can bedriven by transferring charge from the reservoir capacitance 220.

When the system 200 is required to increase the level of charge on thepiezoelectric transducer 210, e.g. to drive the piezoelectric transducer210 to produce a transducer output, the system 200 again operates in twophases.

In a first phase (illustrated in FIG. 2d ) of the charge transferprocess the second and fourth switches 244, 248 are closed, as indicatedby the dashed lines in FIG. 2d , in response to appropriate controlsignals transmitted by the control circuitry 270. A current path istherefore established from the reservoir capacitance 220 through theinductor 230. As current flows through the inductor 230 a magnetic fielddevelops around it, storing energy.

In a second phase of the charge transfer process (illustrated in FIG. 2e), the third and fifth switches 246, 250 are closed, as indicated by thedashed lines in FIG. 2e , and the second and fourth switches 244, 248are opened, in response to appropriate control signals transmitted bythe control circuitry 270. The magnetic field around the inductor 230collapses, inducing a current which flows from the inductor 230 to thepiezoelectric transducer 210, thereby increasing the charge on thepiezoelectric transducer 210.

When the system 200 is required to reduce the level of charge on thepiezoelectric transducer 210, charge can be transferred from thepiezoelectric transducer 210 to the reservoir capacitance 220, such thatthe charge remains available for future use, rather than being lost.This improves the efficiency of the system 200.

The process of transferring charge from the piezoelectric transducer 210to the reservoir capacitance 220 occurs in two phases.

In a first phase (illustrated in FIG. 2f ) the third and fifth switches246, 250 are closed, as indicated by the dashed lines in FIG. 2f , inresponse to appropriate control signals transmitted by the controlcircuitry 270. A current path is therefore established from thepiezoelectric transducer 210 through the inductor 230. As current flowsthrough the inductor 230 a magnetic field develops around it, storingenergy.

In a second phase (illustrated in FIG. 2g ) the second and fourthswitches 244, 248 are closed, as indicated by the dashed lines in FIG.2g , and the third and fifth switches 246, 250 are opened, in responseto appropriate control signals transmitted by the control circuitry 270.The magnetic field around the inductor 230 collapses, inducing a currentwhich flows to the reservoir capacitance 220, thus charging thereservoir capacitance 220.

Thus in the system 200 the piezoelectric transducer 210 can be driven bytransferring charge to it from the reservoir capacitance 220, and chargecan be recycled between the piezoelectric transducer 210 and thereservoir capacitance 220 to improve power efficiency. The power supply260 provides the initial charge to the reservoir capacitance 220 duringthe charging process and occasionally or periodically tops up orrecharges the reservoir capacitance 220 as necessary.

FIG. 2a shows the piezoelectric transducer in a single-endedconfiguration, with a first terminal coupled to the switch network 240and a second terminal coupled to ground or some other reference voltage.FIG. 3a illustrates an alternative system in which a piezoelectrictransducer is coupled in a bridge-tied load configuration.

The system, shown generally at 300 in FIG. 3, includes a piezoelectrictransducer 310, reservoir capacitance 320 for storing charge, aninductor 330 and a switch network 340 (in this example comprising firstto eighth controllable switches 342-356, which may be, for example,MOSFET devices) for transferring charge between the reservoircapacitance 320 and the piezoelectric transducer 310. Again, althoughthe reservoir capacitance 320 is shown in FIG. 3a as a single capacitor,it will be appreciated that the reservoir capacitance 320 mayalternatively be provided by a plurality of capacitors coupled together.

The system 300 also includes a power supply 360 for selectivelyproviding charge to the reservoir capacitance 320. In some examples thepower supply 360 may comprise a battery. In other examples the powersupply 360 may comprise power supply circuitry that receives power froma power supply such as a battery.

The system 300 further includes control circuitry 370, operable tocontrol the switching devices 342-356 to control the transfer of chargebetween the power supply 360, the reservoir capacitance 320 and thepiezoelectric transducer 310, as will now be explained with reference toFIGS. 3b -2 h.

On start-up of the system 300 (or a host device incorporating the system300), charge is transferred from the power supply 360 to the reservoircapacitance 320 to raise a voltage across the reservoir capacitance 320to a level that is suitable for driving the piezoelectric transducer310.

In a first phase of a charging process (illustrated in FIG. 3b ), thefirst and third switches 342, 346 are closed, as indicated by the dashedlines, in response to appropriate control signals transmitted by thecontrol circuitry 370. This creates a current path (indicated by thedashed arrow) though the inductor 330. As current flows through theinductor 330 a magnetic field develops around it, storing energy.

In a second phase of the charging process (illustrated in FIG. 3c ), thefirst and third switches 342, 346 are opened and the second and fourthswitches 344, 348 are closed, as indicated by the dashed lines in FIG.3c , again in response to appropriate control signals transmitted by thecontrol circuitry 370. The magnetic field around the inductor 330collapses, inducing a current which flows from the inductor 330 to thereservoir capacitance 320, thereby charging the reservoir capacitance320.

The first and second phases are repeated until the voltage across thereservoir capacitance 320 has increased to a level that is suitable fordriving the piezoelectric transducer 310, as determined by the controlcircuitry 370 based on a feedback signal received from the piezoelectrictransducer 310. Once the reservoir capacitance 320 has been charged upto the desired level the first switch 342 is opened, thus decoupling thepower supply 360, such that the piezoelectric transducer 310 can bedriven by transferring charge from the reservoir capacitance 320.

When the system 300 is required to increase the level of charge on thepiezoelectric transducer 310 by supplying charge to a first terminal 312of the piezoelectric transducer 310, e.g. to drive the piezoelectrictransducer 310 to produce a transducer output, the system 300 againoperates in two phases.

In a first phase (illustrated in FIG. 3d ) of the charge transferprocess the second and fourth switches 344, 348 are closed, as indicatedby the dashed lines in FIG. 3d , in response to appropriate controlsignals transmitted by the control circuitry 370. A current path istherefore established from the reservoir capacitance 320 through theinductor 330. As current flows through the inductor 330 a magnetic fielddevelops around it, storing energy.

In a second phase of the charge transfer process (illustrated in FIG. 3e), the third, fifth and sixth switches 346, 350, 352 are closed, asindicated by the dashed lines in FIG. 3e , and the second and fourthswitches 344, 348 are opened, in response to appropriate control signalstransmitted by the control circuitry 370. The magnetic field around theinductor 330 collapses, inducing a current which flows from the inductor330 to the first terminal 312 of the piezoelectric transducer 310,thereby increasing the charge on the piezoelectric transducer 310.

When the system 300 is required to increase the level of charge on thepiezoelectric transducer 310 by supplying charge to a second terminal314 of the piezoelectric transducer 310, e.g. to drive the piezoelectrictransducer 310 to produce a transducer output, the system 300 againoperates in two phases.

The first phase is as described above with reference to FIG. 3 d.

In a second phase of the charge transfer process (illustrated in FIG. 3f), the third, seventh and eighth switches 346, 354, 356 are closed, asindicated by the dashed lines in FIG. 3f , and the second and fourthswitches 344, 348 are opened, in response to appropriate control signalstransmitted by the control circuitry 370. The magnetic field around theinductor 330 collapses, inducing a current which flows from the inductor330 to the second terminal 314 of the piezoelectric transducer 310,thereby increasing the charge on the piezoelectric transducer 310.

The process of transferring charge from the piezoelectric transducer 310to the reservoir capacitance 320 occurs in two phases.

In a first phase (illustrated in FIG. 3g ) the third, seventh and eighthswitches 346, 354, 356 are closed, as indicated by the dashed lines inFIG. 2g , in response to appropriate control signals transmitted by thecontrol circuitry 270. A current path is therefore established from thepiezoelectric transducer 310 through the inductor 330. As current flowsthrough the inductor 330 a magnetic field develops around it, storingenergy.

In a second phase (illustrated in FIG. 3h ) the second and fourthswitches 344, 348 are closed, as indicated by the dashed lines in FIG.3h , and the third, seventh and eighth switches 346, 354, 356 areopened, in response to appropriate control signals transmitted by thecontrol circuitry 370. The magnetic field around the inductor 330collapses, inducing a current which flows to the reservoir capacitance320, thus charging the reservoir capacitance 320.

Thus in the system 300 either terminal of the piezoelectric transducer310 can be driven by transferring charge to it from the reservoircapacitance 320, and charge can be recycled between the piezoelectrictransducer 310 and the reservoir capacitance 320 to improve powerefficiency. The power supply 360 provides the initial charge to thereservoir capacitance 320 during the charging process and occasionallyor periodically tops up or recharges the reservoir capacitance 320 asnecessary.

In each of the systems described above, in order to determine thecorrect timing for transfer of charge between the reservoir capacitanceand the piezoelectric transducer and from the power supply to thereservoir capacitance, the voltage across the reservoir capacitance andthe charge stored on the piezoelectric transducer are monitored.

Referring back to FIG. 2a , the control circuitry 270 receives feedbacksignals indicative of a voltage VRES across the reservoir capacitance220, a voltage VPIEZO across the piezoelectric transducer 210 and acurrent IL through the inductor 230.

Similarly, in the system 300 of FIG. 3a , the control circuitry 370receives feedback signals indicative of a voltage VRES across thereservoir capacitance 320, a voltage VPIEZO across the piezoelectrictransducer 310 and a current IL through the inductor 330.

The voltage across a piezoelectric transducer is not an accurateindication of the charge stored by the piezoelectric transducer at highinput signal frequencies, since a piezoelectric transducer is not purelycapacitive, but also includes a resistive component. However at lowerinput signal frequencies (e.g. 100 Hz or less) the voltage across thepiezoelectric transducer is an acceptably accurate indication of thecharge stored by the piezoelectric transducer. At higher frequencies theintegral of the current IL through the inductor 230, 330 is a moreaccurate indication of the charge stored by the piezoelectrictransducer.

Thus, at low input signal frequencies (e.g. when the input signalfrequency is within a first range or below a threshold) the controlcircuitry 270, 370 may use the voltage across the piezoelectrictransducer 210, 310, either directly as an indication of the chargestored on the piezoelectric transducer 210, 310, or to estimate thecharge stored on the piezoelectric transducer 210, 310, in order todetermine whether charge should be transferred to the piezoelectrictransducer 210, 310 from the reservoir capacitance 220, 320 in responseto the input signal received by the control circuitry 270, 370. Athigher input signal frequencies (e.g. when the input signal frequency iswithin a second range or above the threshold) the control circuitry 270,370 may use the current IL through the inductor 230, 330 (or theintegral of the current IL through the inductor 230, 330), eitherdirectly as an indication of the charge stored on the piezoelectrictransducer 210, 310, or to estimate the charge stored on thepiezoelectric transducer 210, 310, in order to determine whether chargeshould be transferred to the piezoelectric transducer 210, 310 from thereservoir capacitance 220, 320 in response to the received input signal.

In the system 200 the control circuitry 270 monitors the charge storedon the piezoelectric transducer 210 (as indicated by the voltage VPIEZO,for low input signal frequencies, and as indicated by the current IL,for higher input signal frequencies) and controls the switch network 240to transfer charge as required between the reservoir capacitance 220 andthe piezoelectric transducer 210.

Thus if the magnitude of the input signal is increasing, such thatcharge must be transferred to the piezoelectric transducer 210 toproduce a suitable output, the switch network 240 can be controlled bythe control circuitry 270 to transfer charge from the reservoircapacitance 220 to the piezoelectric transducer 210. If the magnitude ofthe input signal is decreasing, such that charge must be transferredfrom the piezoelectric transducer 210 to produce a suitable output, theswitch network 240 can be controlled by the control circuitry 270 totransfer charge from the piezoelectric transducer 210 to the reservoircapacitance 220.

The control circuitry 270 also monitors the charged stored on thereservoir capacitance 220 (as indicated by the voltage VRES across thereservoir capacitance) and controls the switch network 240 to transfercharge from the power supply 260 and the reservoir capacitance 220 asrequired to maintain a level of charge on the reservoir capacitance 220required to supply the necessary charge to the piezoelectric transducer210. For example, the control circuitry 270 may periodically compare thelevel of charge on the reservoir capacitance 220 to a threshold, and maycontrol the switch network 240 to transfer charge from the power supply270 to the reservoir capacitance 220 if the level of charge on thereservoir capacitance falls below the threshold, so as to top up orrecharge the reservoir capacitance 220 to compensate for losses, e.g.resistive losses in the switch network 240 or the like, that cause thelevel of charge on the reservoir capacitance 220 to fall over time. Suchtransfer of charge from the power supply 270 to the reservoircapacitance 220 occurs while the switch network 240 is not being used totransfer charge between the reservoir capacitance 220 and thepiezoelectric transducer 210.

The control circuitry 370 of the system 300 operates in a similar mannerto control the switch network 340 to transfer charge between thereservoir capacitance 320 and the piezoelectric transducer 310 and totransfer charge from the power supply 360 to the reservoir capacitance320 as necessary.

In some examples the additional circuitry may be provided to monitor alevel of the input signal (e.g. an amplitude or envelope of the inputsignal) and cause the switch network to transfer charge from the powersupply to the reservoir capacitance based on an expected level of theinput signal, to ensure that sufficient charge is available on thereservoir capacitance to drive the piezoelectric transducer to producean output based on the input signal.

FIG. 4 is a schematic diagram illustrating a system including suchadditional circuitry. The system, shown generally at 400 in FIG. 4, isbased on the system 200 described above with reference to FIG. 2a , andthus those elements that are common to the system 200 are denoted bycommon reference numerals, and will not be described in detail here. Itwill of course be understood by those of ordinary skill in the art thatthe principles describe here are equally applicable to the system 300 ofFIG. 3 a.

The system 400 differs from the system 200 in that it includesadditional circuitry 410 for monitoring a level of the input signal thatis received by the control circuitry 270. The additional circuitry maycomprise, for example, circuitry for monitoring a level or magnitude(e.g. a voltage or current level or magnitude) of the input signal,look-ahead circuitry or envelope detector circuitry. The additionalcircuitry 410 receives the input signal and outputs a signal to thecontrol circuitry 270 indicative of the level (e.g. amplitude orenvelope) of the input signal. The control circuitry 270 controls theswitch network 240, based on the signal received from the additionalcircuitry 410, to transfer charge from the power supply 260 to thereservoir capacitor 260 if necessary, to ensure that sufficient chargeis available on the reservoir capacitor 260 to accommodate an expectedlevel of the input signal, i.e. to drive the piezoelectric transducer210 to produce an output based on the input signal.

The system 400 may further include signal processing circuitry 420configured to process the input signal. The signal processing circuitry420 may include upsampling circuitry to convert the input signal, whichmay be at an audio sampling rate such as 48 kHz, to a higher rate (e.g.12.28 MHz, which is equal to 256 times the sample rate of the inputsignal) that corresponds to an operating frequency of the switch network240 and/or the control circuitry 270. The signal processing circuitry420 may further include interpolation circuitry for interpolating anupsampled version of the input signal.

As will be appreciated by those skilled in the art, such signalprocessing circuitry introduces a delay to the input signal, such thatthe control circuitry 270 receives a delayed version of the input signalin addition to the signal output by the additional circuitry 410. Thesignal processing circuitry 420 may, additionally or alternatively,comprise dedicated delay circuitry to introduce a delay or an additionaldelay to the input signal. This delay is beneficial as it allows timefor the additional circuitry 410 to output the signal used by thecontrol circuitry 270 to control the transfer of charge from the powersupply 260 to the reservoir capacitance 220, if necessary, to ensuresufficient charge is available on the reservoir capacitance 220 toaccommodate the level of the input signal in advance of the delayedinput signal being received by the control circuitry 270.

Thus the additional circuitry 410 provides look-ahead circuitryconfigured to output a control signal to the control circuitry 270 basedon the input signal, to cause the control circuitry 270 to control theswitch network 240 to transfer charge from the power supply 260 to thereservoir capacitance 220, if necessary, to accommodate the inputsignal.

The systems 200, 300 described above use an inductor as an intermediateenergy storage device in a two-stage charge transfer process fortransferring charge from the power supply to the reservoir capacitanceand in a two-stage charge transfer process for transferring chargebetween the reservoir capacitance and the piezoelectric transducer.

In another example, charge may be transferred from the power supply tothe reservoir capacitance and between the reservoir capacitance and thepiezoelectric transducer directly, i.e. without any intermediate energystorage device, as will now be described with reference to FIG. 5, whichschematically illustrates a system 500 for direct charge transfer fordriving a piezoelectric transducer 510.

The system 500 includes a reservoir capacitance 520 for storing charge(which, as in the systems 200, 300 described above, may be provided as asingle capacitor or as multiple capacitors coupled together) and aswitch network 540 (in this example comprising first to thirdcontrollable switches 542-546, which may be, for example, MOSFETdevices) for transferring charge between the reservoir capacitance 520and the piezoelectric transducer 510.

The system, shown generally at 500 in FIG. 5, includes a piezoelectrictransducer 510, a reservoir capacitance 520, switch network circuitry540, a power supply 560 and control circuitry 570.

The system 500 also includes a power supply 560 for selectivelyproviding charge to the reservoir capacitance 520. In some examples thepower supply 560 may comprise power supply circuitry that receives powerfrom a power source such as a battery that is external to the system500. The power supply 560 may be configured to generate an outputvoltage VSUP for charging the reservoir capacitance 520 based on a lowervoltage supplied to the power supply 560 from the external power source.Thus the output voltage VSUP provided by the power supply 560 is greaterthan the supply voltage received by the power supply 560 from theexternal power source, and so the power supply 560 may be referred to asa boosted power supply.

The first switching device 542 is coupled between an output of the powersupply 560 and a first terminal 522 of the reservoir capacitance 520. Asecond terminal 524 of the reservoir capacitance is coupled to aground/reference voltage supply rail.

The second switching device 544 is coupled between the first terminal512 of the reservoir capacitance 520 and the ground/reference voltagesupply rail.

The third switching device 546 is coupled between the first terminal ofthe reservoir capacitance 520 and a first terminal 512 of thepiezoelectric transducer. A second terminal 514 of the piezoelectrictransducer 510 is coupled to the ground/reference voltage supply rail.

The system 500 further includes control circuitry 570, operable tocontrol the switching devices 542-546 to control the transfer of chargebetween the power supply 560, the reservoir capacitance 520 and thepiezoelectric transducer 510 in order to produce an output at thepiezoelectric transducer 510 based on an input signal that is receivedby the control circuitry 570. The control circuitry 570 is coupled tothe first terminal 512 of the piezoelectric transducer 510 so as toreceive a feedback signal indicative of a voltage VPIEZO across thepiezoelectric transducer 510.

The power supply 560 may be a tracking power supply which outputs avoltage VSUP that varies based on a level (e.g. an amplitude or anenvelope) of the input signal or based on a control signal output by thecontrol circuitry 570 to the power supply 560.

In use of the system 500 the power supply 560 outputs a voltage VSUPthat is the same as or slightly higher than the voltage required toproduce a desired output at the piezoelectric transducer 510 based onthe input signal.

In order to produce a desired output (based on the input signal) at thepiezoelectric transducer 510 a first charge transfer operation isperformed, in which the control circuitry 570 outputs appropriatecontrol signals to the switch network 540 to cause the first switch 542to close for a predetermined period of time, while the second and thirdswitches 544, 546 remain open. Thus, charge is transferred from thepower supply 560 to the reservoir capacitance 520. A second chargetransfer operation is then performed, in which the control circuitry 570outputs appropriate control signals to the switch network 540 to causethe third switch 546 to close for a predetermined period of time, whilethe first and second switches 542, 544 remain open. Thus, charge istransferred from the reservoir capacitance 520 to the piezoelectrictransducer 510.

This first and second charge transfer operations are repeated asrequired until the voltage VPIEZO across the piezoelectric transducer510 (as determined by the control circuitry 570 based on the feedbacksignal) reaches the required level, at which point the control circuitry570 transmits control signals to cause the first, second and thirdswitches 542-546 to open.

Transferring charge to the piezoelectric transducer 510 via thereservoir capacitance 520 in this manner has the effect of filtering outany ringing or overshoot in the voltage VSUP output by the power supply560. Thus the switch network 540 and reservoir capacitance 520effectively act as a low pass filter.

Charge can be transferred from the piezoelectric transducer 510 to thereservoir capacitance 520 by transmitting appropriate control signalsfrom the control circuitry 570 to close the third switch 546 and openthe first and second switches 542, 544.

When it is necessary to discharge the reservoir capacitance 520 (e.g. onshut-down of the system, to reduce the risk of potentially harmfulaccidental discharge events) the control circuitry 570 transmits controlsignals to the switch network 540 to open the first and third switches542, 546 and to close the second switch 544, thereby allowing thereservoir capacitance to discharge to the ground/reference voltagesupply rail. Similarly, when it is necessary to discharge thepiezoelectric transducer 510 (e.g. on shut-down of the system, to reducethe risk of potentially harmful accidental discharge events) the controlcircuitry 570 transmits control signals to the switch network 540 toopen the first switch 542 and to close the second and third switches544, 546, thereby allowing the piezoelectric transducer 510 to dischargeto the ground/reference voltage supply rail.

If the reservoir capacitance 520 is small, the first and second chargetransfer operations will need to be repeated a relatively large numberof times in order for the voltage VPIEZO across the piezoelectrictransducer 510 to reach the required level, which in turn requires thatthe control circuitry 570 and the switch network 540 are able to operateat high speed to accommodate high input signal frequencies. However,using a small reservoir capacitance 520 allows accurate control of thevoltage across the piezoelectric transducer 510.

In contrast, a larger reservoir capacitance 520 requires fewerrepetitions of the first and second charge transfer operations andpermits the use of a lower speed switch network 540 and controlcircuitry 570, at the cost of reduced accuracy in the control of thevoltage across the piezoelectric transducer 510.

In some examples the reservoir capacitance 520 may be variable, inresponse to a control signal transmitted by the control circuitry 570(indicated by the dashed arrow in FIG. 5) to allow greater flexibilityin balancing the requirements of accurate control of the voltage VPIEZOand the operating speed of the control circuitry 570 and the switchnetwork 540.

The examples described above with reference to FIGS. 2a , 4 and 5 alldrive a piezoelectric transducer as a single-ended load, i.e. a firstterminal of the piezoelectric transducer is coupled to an output of theswitch network and a second terminal of the piezoelectric transducer iscoupled to the ground/reference voltage supply rail.

It may be advantageous to be able to make the drive bipolar, i.e. driveeither of the terminals of the piezoelectric transducer. This can beachieved by using commutator circuitry coupled to the piezoelectrictransducer, as will now be described with reference to FIG. 6, whichshows a system 600 for driving a piezoelectric transducer 610.

The system 600 includes a reservoir capacitance 620, a switch network650, power supply circuitry 660 and control circuitry 670 which may besimilar to the corresponding elements of the system 200 of FIG. 2a , ormay be similar to the corresponding elements of the system 500 of FIG.5. Thus the structure and operation of the switch network 650, powersupply circuitry 660 and control circuitry 670 will not be describedhere in detail.

The system 600 further includes commutator circuitry 680, which in theillustrated example includes first to fourth controllable switches682-688. The commutator circuitry 680 is coupled to the controlcircuitry 670 so as to receive control signals for controlling theoperation of the controllable switches 682-688 according to the inputsignal.

The first controllable switch 682 is coupled between a first node 690 ofthe commutator circuitry 680 and a first terminal 612 of thepiezoelectric transducer 610. The first node 690 of the commutatorcircuitry 680 is coupled to an output of the switch network 640, whichis operative to transfer charge between the reservoir capacitance 620and the piezoelectric transducer 610, either via an intermediateinductor (as in the system 200 of FIG. 2a , described above), ordirectly (i.e. without any intermediate inductor, as in the system 500of FIG. 5, described above).

The second controllable switch 684 is coupled between the first terminal612 of the piezoelectric transducer 610 and the ground/reference voltagesupply rail.

The third controllable switch 686 is coupled between the first node 690of the commutator circuitry 680 and a second terminal 614 of thepiezoelectric transducer 610.

The fourth controllable switch 688 is coupled between the secondterminal 614 of the piezoelectric transducer 610 and theground/reference voltage supply rail.

By selectively opening and closing the controllable switches 682-688 oneof the first and second terminals 612, 614 of the piezoelectrictransducer 610 can be coupled to the output of the switch network 640,and the other of the first and second terminals 612, 614 of thepiezoelectric transducer 610 can be coupled to the ground/referencevoltage supply rail.

The operation of the controllable switches 682-688 is controlled by thecontrol circuitry 670 according to the polarity (with respect to groundor the reference voltage supplied by the ground/reference voltage supplyrail) of the input signal, as shown in the table below.

Input signal First Second Third Fourth polarity switch 682 switch 684switch 686 switch 688 Positive Closed Open Open Closed Negative OpenClosed Closed Open

Thus, when the input signal is positive, the fourth switch 688 isclosed, thereby coupling the second terminal 614 of the piezoelectrictransducer 610 to the reference voltage supply rail. The second andthird switches 684, 686 are open. When charge is to be transferred tothe first terminal 612 of the piezoelectric transducer 610, the firstswitch 682 is closed, thus coupling the first terminal 612 of thepiezoelectric transducer 610 to the output of the switch network 640.

When the input signal is negative, the second switch 884 is closed,thereby coupling the first terminal 612 of the piezoelectric transducer610 to the ground/reference voltage supply rail. The first and fourthswitches 682, 688 are open. When charge is to be transferred to thesecond terminal 614 of the piezoelectric transducer 610, the thirdswitch 686 is closed, thus coupling the second terminal 614 of thepiezoelectric transducer 610 to the output of the switch network 640.

Thus either the first terminal 612 or the second terminal 614 of thepiezoelectric transducer 610 can be driven by the charge transferred bythe switch network 640, depending on the polarity of the input signal.

As will apparent from the foregoing discussion, the circuitry of thepresent disclosure provides a power efficient means for driving apiezoelectric transducer using charge, which reduces the hysteresis andcreep that can arise when such a piezoelectric transducer isvoltage-driven. Improved power efficiency arises due to the recycling ofcharge between the reservoir capacitance and the piezoelectrictransducer, and the use of the reservoir capacitor helps to reduce theeffects of ringing or overshoot in a voltage output by a power supplythat is used to charge the reservoir capacitance.

Embodiments may be implemented as an integrated circuit which in someexamples could be a codec or audio DSP or similar. Embodiments may beincorporated in an electronic device, which may for example be aportable device and/or a device operable with battery power. The devicecould be a communication device such as a mobile telephone or smartphoneor similar. The device could be a computing device such as a notebook,laptop or tablet computing device. The device could be a wearable devicesuch as a smartwatch. The device could be a device with voice control oractivation functionality such as a smart speaker. In some instances thedevice could be an accessory device such as a headset, headphones,earphones, earbuds or the like to be used with some other product.

The skilled person will recognise that some aspects of theabove-described apparatus and methods, for example the discovery andconfiguration methods may be embodied as processor control code, forexample on a non-volatile carrier medium such as a disk, CD- or DVD-ROM,programmed memory such as read only memory (Firmware), or on a datacarrier such as an optical or electrical signal carrier. For manyapplications, embodiments will be implemented on a DSP (Digital SignalProcessor), ASIC (Application Specific Integrated Circuit) or FPGA(Field Programmable Gate Array). Thus the code may comprise conventionalprogram code or microcode or, for example code for setting up orcontrolling an ASIC or FPGA. The code may also comprise code fordynamically configuring re-configurable apparatus such asre-programmable logic gate arrays. Similarly the code may comprise codefor a hardware description language such as Verilog™ or VHDL (Very highspeed integrated circuit Hardware Description Language). As the skilledperson will appreciate, the code may be distributed between a pluralityof coupled components in communication with one another. Whereappropriate, the embodiments may also be implemented using code runningon a field-(re)programmable analogue array or similar device in order toconfigure analogue hardware.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope.

1. Circuitry for driving a piezoelectric transducer based on an inputsignal, the circuitry comprising: a power supply; a reservoircapacitance; switch network circuitry; and control circuitry configuredto control operation of the switch network circuitry so as to charge thereservoir capacitance from the power supply and to transfer chargebetween the reservoir capacitance and the piezoelectric transducer. 2.Circuitry according to claim 1 further comprising one or more inductors,wherein the control circuitry is configured to control operation of theswitch network circuitry to transfer charge between the reservoircapacitance and the piezoelectric transducer via one of the one or moreinductors.
 3. Circuitry according to claim 2 wherein the controlcircuitry is configured to control operation of the switch networkcircuitry to transfer charge from the power supply to the reservoircapacitance via one of the one or more inductors.
 4. Circuitry accordingto claim 1 wherein the control circuitry is configured to controloperation of the switch network circuitry so as to transfer chargedirectly between the reservoir capacitance and the piezoelectrictransducer.
 5. Circuitry according to claim 4 wherein a capacitancevalue of the reservoir capacitance is variable.
 6. Circuitry accordingto claim 1 wherein the control circuitry is further configured tocontrol operation of the switch network to transfer charge from thepower supply to the reservoir capacitance based on an indication of alevel of the input signal.
 7. Circuitry according to claim 6 wherein thecircuitry further comprises monitoring circuitry configured to monitor alevel or magnitude of the input signal and to output a signal indicativeof the level or magnitude of the input signal to the control circuitry.8. Circuitry according to claim 6 wherein the circuitry furthercomprises look-ahead circuitry configured to receive the input signaland to output a signal indicative of the level of the input signal tothe control circuitry.
 9. Circuitry according to claim 6 wherein thecircuitry further comprises envelope detector circuitry configured toreceive the input signal and to output a signal indicative of anenvelope of the input signal to the control circuitry.
 10. Circuitryaccording to claim 1 wherein the switch network circuitry is configuredfor coupling to a single terminal of the piezoelectric transducer. 11.Circuitry according to claim 1 wherein the switch network circuitry isconfigured to be coupled to first and second terminals of thepiezoelectric transducer.
 12. Circuitry according to claim 1 wherein thepower supply is configured to provide an output voltage that variesaccording to a level of the input signal.
 13. Circuitry according toclaim 1 wherein the power supply is configured to provide an outputvoltage that is greater than a voltage supplied to the power supply by apower source.
 14. Circuitry according to claim 1 wherein the powersupply comprises a switching power supply.
 15. Circuitry according toclaim 1 wherein the control circuitry is configured to receive afeedback signal indicative of a level of charge of the piezoelectrictransducer.
 16. Circuitry according to claim 15 wherein the feedbacksignal is based on a voltage across the piezoelectric transducer. 17.Circuitry according to claim 15, wherein the circuitry comprises one ormore inductors, wherein the control circuitry is configured to controloperation of the switch network circuitry to transfer charge between thereservoir capacitance and the piezoelectric transducer via one of theone or more inductors, and wherein the feedback signal is based on acurrent through the one of the one or more inductors.
 18. Circuitryaccording to claim 1 wherein the input signal comprises an audio signal,a haptic waveform or an ultrasonic signal.
 19. Circuitry according toclaim 1 further comprising commutator circuitry coupled to the switchnetwork circuitry, the commutator circuitry being configured toselectively couple a first or a second terminal of the piezoelectrictransducer to an output of the switch network circuitry.
 20. Circuitryfor driving a piezoelectric transducer based on an input signal, thecircuitry comprising: a power supply; a reservoir capacitance; switchnetwork circuitry; and control circuitry configured to control operationof the switch network circuitry so as to charge the reservoircapacitance from the power supply based on an indication of the inputsignal.
 21. Circuitry for estimating a level of charge on apiezoelectric transducer, the circuitry comprising: an inductor fortransferring charge to the piezoelectric transducer from a chargesource; control circuitry configured to control transfer of charge tothe piezoelectric transducer based on an input signal, wherein thecontrol circuitry is configured to receive an indication of a currentthrough the inductor and an indication of a voltage across thepiezoelectric transducer, and wherein the control circuitry isconfigured to estimate the level of charge on the piezoelectrictransducer based on: the indication of the voltage across thepiezoelectric transducer when a frequency of the input signal is withina first range, and the indication of the current through the inductorwhen the frequency of the input signal is within a second range. 22.Integrated circuitry comprising the circuitry of claim
 1. 23. A devicecomprising the circuitry of claim
 1. 24. A device according to claim 23,wherein the device comprises a mobile telephone, a tablet or laptopcomputer, a smart speaker, an accessory device, headphones, earphones orearbuds.